Image forming apparatuses having a recording head to eject droplets such as ink droplets have been used for printers, facsimiles, copiers, and multifunctional machines, and specific examples thereof include inkjet recording devices. Such image forming apparatuses record images by ejecting droplets such as ink droplets from a recording head toward a recording material such as paper sheets, overhead projection (OHP) sheets, and other materials to which an ink can be adhered, to form an ink image on the recording material. Such image forming apparatuses are broadly classified into serial image forming apparatuses in which a recording head ejects droplets while moving in a main scanning direction to form an image on a recording material fed in a sub-scanning direction, and line image forming apparatuses having a fixed line recording head ejecting droplets on a recording material fed in a direction perpendicular to the line recording head.
In this application, image forming apparatuses mean apparatuses which eject droplets so as to be adhered to a recording material such as paper, yarn, fiber, fabric, leather, metal, plastic, glass, wood, and ceramic to form an image thereon. In addition, image formation means not only formation of a meaningful image such as letters and figures but also formation of a meaningless image such as patterns (i.e., mere adhesion of droplets on a recording material). Further, ink means not only so-called inks but also other liquids for use in image formation such as recording liquids, fixing liquids, and other liquids (e.g., DNA samples, photoresist liquids, patterning liquids, and liquid resins). Furthermore, image means not only two-dimensional images but also images formed on a three-dimensional object and three dimensional images themselves formed by ink.
There is an image forming apparatus which includes a recording head, a head tank (i.e., sub-tank, or buffer tank) to supply an ink to the recording head, and a main tank (i.e., ink cartridge) which is detachably attached to the main body of the image forming apparatus while containing the ink therein and which feeds the ink to the head tank through a tube using a pump.
In such an image forming apparatus having a head tank, when a used main tank (i.e., ink cartridge) is detached from the image forming apparatus, a problem (hereinafter referred to as an air suction problem) in that a small amount of air enters into the tube connecting the main tank with the head tank is often caused. This air suction problem is also caused when the tube has high air permeability.
The air bubbles thus formed in the tube (i.e., ink passage) are fed to the head tank and stays in the head tank as the ink in the head tank is consumed for recording images. In this regard, the air bubbles tend to stay in an upper portion of the head tank due to an ascending force for the bubbles. In addition, when the image forming apparatus has a filter member between the head tank and the recording head, the air bubbles tend to remain in the head tank because of being unable to pass through the filter.
When such a head tank containing air bubbles therein is set under a high temperature condition, the air bubbles expand, thereby increasing the internal pressure of the head tank. In general, nozzles of a recording head have a negative pressure so that the ink therein has meniscus so as not to drop from the nozzles. However, when the internal pressure of the head tank increases, the nozzles cannot maintain the negative pressure, thereby making it impossible to perform normal image formation.
In attempting to discharge air bubbles from a head tank, there is a choking method in which the ink in the ink cartridge is pressed while a valve is formed on a portion of the tube connecting the main tank with the head tank; the surface of the nozzles of the recording head is capped while closing the valve; the ink is discharged from the nozzles using a suction pump to decrease the internal pressure of the head tank; and the valve is opened to form large pressure difference, thereby discharging the air bubbles in the head tank from the nozzles.
However, this choke method has a drawback in that since the air discharging operation is performed at once utilizing the large pressure difference, a large amount of ink is discharged together with air, thereby incurring waste.
In addition, there is a proposal for a method in which an ink level sensor, an air discharging hole, and a valve (non-return valve) are provided on upper portions of a head tank, and the air discharging hole is covered with a discharging cap connected with a discharging tube pump to discharge air bubbles from the head tank using the tube pump.
This method has a drawback in that it is necessary to perform control such that the flow path connected with the discharging cap and the flow path connected with a cap covering the nozzle surface have to be switched with each other, and in addition two ink level sensors are necessary, thereby increasing the number of parts, resulting in increase of the costs of the apparatus.
For these reasons, the inventors recognized that there is a need for an image forming apparatus in which air bubbles can be discharged from the head tank by a simple mechanism without performing complex control while reducing the amount of the discharged ink.